The aim of this article is to clarify the role of the "empirical concept of matter" in Kant's plan of metaphysics of nature by considering the background in the history of philosophy and science. Kant divides metaphysics of nature into "transcendental part", or general metaphysics, which is also called ontology, and special metaphysical natural science called "metaphysics of corporeal nature", for which the empirical concept of matter is required as its ground. Kant explains that metaphysics of nature must contain the metaphysical foundations of "concept of motion, of impenetrability (the filling of space), of inertia, and so on", to make possible the application of mathematics to natural science. This concept is a bridge between general and special (corporeal nature's) metaphysics. For it is by adding this empirical concept to the general, "transcendental" principle (for example, "all change has cause") that we obtain the concrete, "(special) metaphysical" one, such as the proposition "all change (of states of matter) has an external cause", from the mechanics chapter of Metaphysical Foundations of Natural Science as the third law (proposition 3). It is important to acknowledge the difference that Kant treats the laws of mechanics as propositions (for which demonstration is required), while Newton's laws of motion were axioms (from which many propositions are demonstrated). But Kant sees a certain metaphysical postulate even in Newton's mathematical and experimental position. He says that Newton must have had metaphysical views of matter in mind, otherwise he could not argue for a universal property of matter such as gravitation. His intention is to lay further foundations for Newton's tacit postulate. The empirical concept of matter is a task for which the metaphysical foundations must be established, which is the ground of such application of mathematics as Newton attempted.